Since their introduction in the early 1980s, microchannel heat sinks have shown much potential for high heat-flux cooling applications and have been used in the industry. However, existing microchannels include conventional parallel channel arrangements which are used are not well suited for cooling heat producing devices which have spatially-varying heat loads. Such heat producing devices have areas which produce more heat than others. These hotter areas are hereby designated as “hot spots” whereas the areas of the heat source which do not produce as much heat are hereby termed, “warm spots”.
FIG. 1A illustrates a prior art heat exchanger 10 which is coupled to an electronic device 99, such as a microprocessor via a thermal interface material 98. As shown in FIG. 1A, fluid generally flows from a single inlet port 12 and flows along the bottom surface 11 in between the parallel microchannels 14, as shown by the arrows, and exits through the outlet port 16. Although the heat exchanger 10 cools the electronic device 99, the fluid flows from the inlet port 12 to the outlet port 16 in a uniform manner. In other words, the fluid flows substantially uniformly along the entire bottom surface 11 of the heat exchanger 10 and does not supply more fluid to areas in the bottom surface 11 which correspond with hot spots in the device 99. In addition, the temperature of liquid flowing from the inlet generally increases as it flows along the bottom surface 11 of the heat exchanger. Therefore, regions of the heat source 99 which are downstream or near the outlet port 16 are not supplied with cool fluid, but actually fluid which has already been heated upstream. In effect, the heated fluid actually propagates the heat across the entire bottom surface 11 of the heat exchanger and region of the heat source 99, whereby fluid near the outlet port 16 is so hot that it becomes ineffective in cooling heat source. In addition, the heat exchanger 10 having only one inlet 12 and one outlet 16 forces fluid to travel along the long parallel microchannels 14 in the bottom surface 11 for the entire length of the heat exchanger 10, thereby creating a large pressure drop.
FIG. 1B illustrates a side view diagram of a prior art multi-level heat exchanger 20. Fluid enters the multi-level heat exchanger 20 through the port 22 and travels downward through multiple jets 28 in the middle layer 26 to the bottom surface 27 and out port 24. In addition, the fluid traveling along the jets 28 may or may not uniformly flow down to the bottom surface 27. Nonetheless, although the fluid entering the heat exchanger 20 is spread over the length of the heat exchanger 20, the design does not provide more fluid to the hotter areas of the heat exchanger 20 and heat source that are in need of more fluid flow circulation.
In addition, conventional heat exchangers are made of materials which have high thermal resistance in the bottom surface, such that the heat exchanger has a coefficient of thermal expansion which matches that of the heat source 99. The high thermal resistance of the heat exchanger thereby does not allow sufficient heat exchange with the heat source 99. To account for the high thermal resistance, larger channel cross-sectional areas are chosen such that more thermal exchange occurs between the heat exchanger 10 and the heat source 99. In addition, the dimensions of the channels in the heat exchanger are scaled down and the distance between the channel walls and the hydraulic diameter is made smaller, the thermal resistance of the heat exchanger is reduced. However, a problem with using narrow microchannels is the increase in pressure drop along the channels. The increase in pressure drop places extreme demands on a pump driving the fluid through the heat exchanger. In addition, larger microchannel dimensions also cause a larger pressure drop between the inlet and outlet ports, due to the long distance that one or two phase fluid must travel. Further, boiling of the fluid in a microchannel heat exchanger causes a larger pressure drop for a given flowrate due to the mixing of fluid and vapor as well as the acceleration of the fluid into the vapor phase. Both of these factors increase the pressure drop per unit length. The large pressure drop created within the current microchannel heat exchangers require larger pumps which can handle higher pressures and thereby are not feasible in a microchannel setting.
What is needed is a microchannel heat exchanger which is configured to achieve proper temperature uniformity in the heat source. What is also needed is a heat exchanger which is configured to achieve proper uniformity in light of hot spots in the heat source. What is also needed is a heat exchanger having a relatively high thermal conductivity to adequately perform thermal exchange with the heat source. What is further needed is a heat exchanger which is configured to achieve a small pressure drop between the inlet and outlet fluid ports.